Forging method

ABSTRACT

Provided is a forging method for highly increasing hardness of a forged product as well as suppressing a decrease in the hardness in a use environment. The forging method includes a warm-forging step (S 105 ) of warm-forging a metallic workpiece  10  which has been solutionized; and an artificial aging step (S 106 ) of artificially aging the workpiece  10  after the warm-forging step (S 105 ) in advance, at equal to an environmental temperature or higher than a temperature at which a product of the workpiece is used after production. Here, an equivalent plastic strain of the workpiece that increases between before and after the warm-forging step is in the range from 0.1 to 2.5.

FIELD OF INVENTION

The present invention relates to a forging method.

BACKGROUND ART

In recent development of automotive vehicles, a component with a lighter weight and a higher strength has been developed to improve fuel efficiency. For example, Patent Document 1 discloses technology for producing a forged product made of an aluminum alloy via warm-forging in order to highly increase a strength of the forged product (See Patent Document 1).

DOCUMENTS OF PRIOR ART Patent Documents

-   Patent Document 1: Japanese Unexamined Patent Application     Publication No. 2010-137284

SUMMARY OF INVENTION Problems to be Solved by Invention

However, when a forged product is left to stand after warm-forging, the forged product deteriorates with time failing to keep stable hardness in a use environment of a vehicle, leading to considerable defects.

Therefore, an object of the present invention is to provide a forging method for highly increasing a strength of a forged product as well as suppressing decrease in hardness thereof in a use environment.

Means for Solving Problems

For solving the above defects, the present invention is directed to a forging method including a warm-forging step of warm-forging a metallic workpiece which has been solutionized, at a recrystallization temperature or below; and an artificial aging step to be performed after the warm-forging step so that the workpiece is artificially aged in advance after production at a temperature in a use environment or above.

Here, the warm-forging means a process of forging and molding a metallic workpiece by using a mold under the conditions in which the metallic workpiece is heated at the temperature at which structure of the metallic workpiece recrystallizes (i.e., recrystallization temperature) or below.

The above method may enhance precipitation strengthening via the artificial aging process, which highly increases a strength of the forged product (i.e., workpiece) as well as suppresses change in hardness of the forged product in use after production.

Here, at the warm-forging step, an equivalent plastic strain that increases between before and after the warm-forging process may be preferably in the range from 0.1 to 2.5.

The equivalent plastic strain that increases between before and after the forging is calculated, for example, by the CAE (Computer Aided Engineering) analysis.

Further, at the warm-forging step, more preferably an equivalent plastic strain that increases between before and after the warm-forging process may be in the range from 0.4 to 2.1.

Effect of Invention

According to the present invention, a forging method for highly increasing a strength of a forged product as well as suppressing aging deterioration thereof in a use environment may be provided.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a process chart of a forging method of a present embodiment.

FIGS. 2A-2D are diagrams respectively showing a tie-rod as an example of a workpiece prepared in the forging method of the present embodiment. Specifically, FIG. 2A shows a tie-rod after cut off; FIG. 2B shows the tie-rod after cold-forged (i.e., preliminary molding); FIG. 2C shows the tie-rod after warm-forged; and FIG. 2D shows the tie-rod after deburred.

FIG. 3 is a microphotograph of a workpiece with the equivalent plastic strain of 0.42.

FIG. 4 is a microphotograph of a workpiece with the equivalent plastic strain of 1.39.

FIG. 5 is a microphotograph of a workpiece with equivalent plastic strain of 2.07.

FIG. 6 is a microphotograph of a workpiece with the equivalent plastic strain of 2.66.

FIG. 7 is a graphic diagram showing an effect of the forging method of the present embodiment by the relationships between a rolling reduction rate and an elongation rate.

FIG. 8 is a graphic diagram showing an effect of the forging method of the present embodiment by the relationships between an equivalent plastic strain and a tensile strength.

FIG. 9 is a graphic diagram showing an effect of the forging method of the present embodiment.

FIG. 10 is a graphic diagram showing an effect of the forging method of the present embodiment by the relationships between a working time and hardness.

FIG. 11 is a graphic diagram showing an effect of the forging method of the comparative example by the relationships between a working time and hardness.

EMBODIMENTS FOR CARRYING OUT INVENTION

Hereinafter, an embodiment of the present invention will be described in detail referring to the attached drawings of FIGS. 1-11.

As shown in FIG. 1, the forging method of the present embodiment includes a warm-forging step (S105) of molding a workpiece 10 via warm-forging, and an artificial aging step (S106) of artificially aging the workpiece having been warm-forged so as to highly increase a strength of a forged product and prevent the aging deterioration thereof in an use environment of a vehicle. Here, a case in which the workpiece 10 is made of an aluminum alloy will be exemplified.

Further, as shown in FIGS. 2A-2D, exemplified is a case in which the workpiece 10 is a substantially rod shaped tie-rod which turns a knuckle rotatably supporting a wheel. Hence, the workpiece 10 (or tie-rod) thus produced includes a semispherical boss 11 externally fitted to a ball joint of the knuckle, a round rod-like shaft 12 connected to a rod at an actuator side, and a neck 13 formed between the boss 11 and the shaft 12.

Substantially, the outer diameter in the workpiece 10 becomes smaller in the order of the shaft 12, the boss 11, and the neck 13. Further, the equivalent plastic strain becomes larger in the order of the shaft 12 (e.g., 0.2), the boss 13 (e.g., 0.9), the neck 11 (e.g., 2.2) relative to the workpiece 10 before generating strains (i.e., before forging) (see FIG. 9).

<Cutting Off Step: S101>

In the step of S101, a workpiece 10 with an appropriate size is cut off from a raw material made of an aluminum alloy (see FIG. 2A).

<Cold-Forging Step: S102>

In the step of S102, the workpiece 10 thus cut off is cold-forged and preliminary molded (see FIG. 2B). Herein, the cold-forging is a process of forging the workpiece 10 at a low temperature (e.g., an ambient temperature (about 25° C.) or below) which is the recrystallization temperature of the aluminum alloy or below.

<Solution Treatment Step: S103>

In the step of S103, the workpiece 10 obtained after the cold-forging step is solutionized. More specifically, the workpiece 10 is heated up to a solution treatment temperature (e.g., 540° C.) in an appropriate furnace so that alloy components in the workpiece 10 are solutionized to facilitate aging precipitation and also remove the strains generated by the cold-forging.

<Warm-Heating Step: S104>

In the step of S104, the workpiece 10 obtained after the solution treatment is heated to a warm-heating temperature so as to be subjected to a warm-forging step. The warm-heating temperature may be in the range from an ambient temperature to a recrystallization temperature. More specifically, in the present embodiment, since the workpiece 10 is made of an aluminum alloy, the warm-heating temperature may be, for example, in the range from 100° C. to the recrystallization temperature of the alloy.

<Warm-Forging Step: S105>

In the step of S105, the workpiece 10 kept heated at the warm-heating temperature (i.e., recrystallization temperature or below) is warm-forged (FIG. 2C).

The warm-forging step is performed in such a way that an equivalent plastic strain of the workpiece 10 in the forging direction (i.e., compression direction) may be in the normal range from 0.1 to 2.5.

If the equivalent plastic strain becomes higher than 2.5, a tensile strength of the workpiece 10 after production becomes lower. This is because, a part of the dislocations structure formed by the forging forms dislocation cell structures (i.e., recrystallization process)(see FIG. 6).

Here, as shown in FIG. 8, preferably the equivalent plastic strain is in the suitable range from 0.1 to 2.1 (see also FIGS. 3-5). Further, the equivalent plastic strain is preferably in the most suitable range from 0.4 to 2.1. The equivalent plastic strain in the above range allows the tensile strength to become excellently high.

FIG. 8 shows the following relationships. Namely, as the equivalent plastic strain becomes larger, a dislocation density in the workpiece 10 gradually becomes higher, and a tensile strength gradually becomes higher.

FIG. 7 shows that the elongation (%) of the workpiece 10 after production tends to become smaller as the equivalent plastic strain becomes larger. Note, when the elongation becomes smaller, toughness of the workpiece 10 becomes lower, which makes the workpiece 10 brittle. Further, given the tensile strength of the hot-forged product of a general aluminum alloy is in the range from 285 to 385 Mpa, it is shown that even when the equivalent plastic strain is near 0.1, a hot-forged product of the present invention has a tensile strength near the upper limit value of the hot-forged product of a general aluminum alloy (see FIG. 8)

<Artificial Aging Step: S106>

In the step of S106, the workpiece 10 obtained after warm-forging is artificially aged. More specifically, the workpiece 10 is artificially aged in advance at a predetermined artificial aging temperature for a predetermined aging time so that the product (i.e., workpiece 10) with a high strength does not cause aging deterioration in use after production.

The predetermined artificial aging temperature is set to a temperature equal to or higher than the environmental temperature at which the product (i.e., workpiece 10) after production is used. This may suppress the aging deterioration of the product (i.e., workpiece 10) in use after production. For example, when the product (i.e., workpiece 10) after production is a tie-rod, the predetermined artificial aging temperature is set in the range from 150 to 200° C. (see FIG. 10).

The predetermined artificial aging time is determined by experiments conducted in advance, and set to as short a time as possible within the range preventing the aging deterioration after production.

<Deburring (Trimming) Step>

In the step of S107, the workpiece 10 thus artificially aged is subjected to the deburring (or trimming) step (see FIG. 2D). More specifically, a burr 14 of the workpiece 10 formed at the warm-forging step is cut off.

<Finishing Step>

In the step of S108, the workpiece 10 obtained after deburring is finished. More specifically, for example, a surface of the workpiece 10 is ground and cleaned.

The above described forging method enables an increase in the hardness of the workpiece 10 by artificially aging the workpiece 10 obtained after warm-forging (see FIG. 9). That is, the artificial aging performed on the workpiece 10 prevents a change or a decrease in the hardness of the product (i.e., workpiece 10) while the product is used after production (see FIG. 10).

On the contrary, when the product (i.e., workpiece 10) is not artificially aged after the warm-forging step and used as it is, the aging deterioration progresses while the product (i.e., workpiece 10) is used. As a result, the hardness of the product may not be stably kept, and thus be changed (see Comparative Example in FIG. 9 and FIG. 11).

Hereinbefore, an embodiment of the present invention has been described. However, the present invention is not limited to the embodiment, and free of suitable modifications.

Further, it should be noted that in the above described embodiment, a case that the workpiece 10 is made of an aluminum alloy is exemplified. However, the workpiece 10 may be made of other kinds of metals.

DESCRIPTION OF REFERENCE NUMBERS

-   -   10 Workpiece     -   11 Boss     -   12 Shaft     -   13 Neck     -   14 Burr 

1. A forging method comprising: a warm-forging step of warm-forging a metallic workpiece which has been solutionized, at a recrystallization temperature or below; and an artificial aging step of artificially aging the workpiece in advance after the warm-forging step, at equal to an environmental temperature or higher than a temperature at which a product of the workpiece is used after production.
 2. The forging method described in claim 1, wherein an equivalent plastic strain that increases between before and after the warm-forging step is in the range from 0.1 to 2.5.
 3. The forging method described in claim 1, wherein an equivalent plastic strain that increases between before and after the warm-forging step is in the range from 0.4 to 2.1. 